1. Technical Field
The present disclosure relates to a microelectromechanical structure, in particular a gyroscope sensitive to yaw angular accelerations, having enhanced mechanical characteristics, in particular in the rejection of acceleration noise.
2. Description of the Related Art
As is known, micromachining techniques enable manufacturing of microelectromechanical structures or systems (MEMS) within layers of semiconductor material, which have been deposited (for example, a layer of polycrystalline silicon) or grown (for example, an epitaxial layer) on sacrificial layers, which are removed by chemical etching. Inertial sensors, accelerometers, and gyroscopes built using this technology are having a growing success, for example, in the automotive field, in inertial navigation, or in the sector of portable devices.
In particular, known to the art are integrated gyroscopes made of semiconductor material built using MEMS technology.
These gyroscopes operate on the basis of the theorem of relative accelerations, exploiting the Coriolis acceleration. When an angular velocity is applied to a mobile mass that moves with a linear velocity, the mobile mass “feels” an apparent force, referred to as the “Coriolis force”, which determines a displacement in a direction perpendicular to a direction of the linear velocity and to an axis about which the angular velocity is applied. The mobile mass is supported via springs that enable its displacement in the direction of the apparent force. On the basis of Hooke's law, the displacement is proportional to the apparent force so that, from the displacement of the mobile mass, it is possible to detect the Coriolis force and a value of the angular velocity that has generated it. The displacement of the mobile mass can, for example, be detected capacitively, by determining, in resonance conditions, capacitance variations caused by movement of mobile electrodes, which are fixed with respect to the mobile mass and are comb-fingered with fixed electrodes.
Published U.S. Patent Application Nos. US2007/0214883, US2009/0064780, and US2009/0100930, filed by the present applicant, disclose a microelectromechanical integrated sensor with rotary driving movement and sensitive to yaw angular velocities.
The microelectromechanical sensor comprises a single driving mass, anchored to a substrate and actuated with rotary motion. Through openings are provided within the driving mass, and corresponding sensing masses are set in the through openings; the sensing masses are enclosed within the overall dimensions of the driving mass, are suspended with respect to the substrate, and are connected to the driving mass via flexible elements. Each sensing mass is fixed with respect to the driving mass during the rotary motion, and has a further degree of freedom of movement as a function of an external stress, in particular a Coriolis force, acting on the sensor. The flexible elements, thanks to their particular construction, enable the sensing masses to perform a linear movement of detection in a radial direction belonging to the plane of the sensor, in response to a Coriolis acceleration. This movement of detection is substantially uncoupled from the actuation movement of the driving mass. The microelectromechanical structure, in addition to being compact (in so far as it envisages a single driving mass enclosing in its overall dimensions a number of sensing masses), enables, with minor structural modifications, a uniaxial gyroscope, a biaxial gyroscope, or a triaxial gyroscope (and/or possibly an accelerometer, according to the electrical connections implemented) to be obtained, at the same time ensuring an excellent uncoupling of the driving dynamics from the detection dynamics.
FIG. 1 shows an exemplary embodiment of a uniaxial microelectromechanical gyroscope, designated by 1, provided according to the teachings contained in the aforesaid patent applications.
The gyroscope 1 is provided in a die 2, comprising a substrate 2a made of semiconductor material (for example, silicon), and a frame 2b; the frame 2b defines inside it an open region 2c, which overlies the substrate 2a and is designed to house detection structure of the gyroscope 1 (as described in detail hereinafter). The open region 2c has a generally square or rectangular configuration in a horizontal plane (in what follows, plane of the sensor xy), defined by a first horizontal axis x and by a second horizontal axis y, which are fixed with respect to the die 2; the frame 2b has sides substantially parallel to the horizontal axes x, y. Die pads 2d are arranged along one side of the frame 2b, aligned, for example, along the first horizontal axis x. In a way not illustrated, the die pads 2d enable the detection structure of the gyroscope 1 to be electrically contacted from the outside.
The gyroscope 1 comprises a driving structure, housed within the open region 2c and formed by a driving mass 3 and by a driving assembly 4.
The driving mass 3 has, for example, a generally circular geometry with radial symmetry, with a substantially planar configuration with main extension in the plane of the sensor xy, and negligible dimension, with respect to the main extension, in a direction parallel to a vertical axis z, forming with the first and second horizontal axes x, y a set of three orthogonal axes, fixed with respect to the die 2. The driving mass 3 defines at a central empty space 6, a center O of which coincides with the centroid and a center of symmetry of the entire structure.
The driving mass 3 is anchored to the substrate 2a by means of a first anchorage 7a set at the center O, to which it is connected through first elastic anchorage elements 8a. The driving mass 3 is possibly anchored to the substrate 2a by means of further anchorages (not illustrated), set outside the same driving mass 3, to which it is connected by means of further elastic anchorage elements (not illustrated), for example, of the folded type. The first and further elastic anchorage elements enable a rotary movement of the driving mass 3 about an axis of actuation passing through the center O, parallel to the vertical axis z and perpendicular to the plane of the sensor xy, with a driving angular velocity {right arrow over (Ω)}a.
The driving mass 3 has a pair of through openings 9a, 9b, aligned in a radial direction, for example, along the second horizontal axis y, and set on opposite sides with respect to the empty space 6; the through openings 9a, 9b have in plan view a substantially rectangular shape, with main extension in a direction transverse to the radial direction.
The driving assembly 4 comprises a plurality of driven arms 10, extending externally from the driving mass 3 in a radial direction and arranged at equal angular distances apart, and a plurality of first and second driving arms 12a, 12b, extending parallel to, and on opposite sides of, respective driven arms 10. Each driven arm 10 carries a plurality of first electrodes 13, extending perpendicular to, and on both sides of, the same driven arm 10. Furthermore, each of the first and second driving arms 12a, 12b carries respective second electrodes 14a, 14b, extending towards the respective driven arm 10, and comb-fingered with the corresponding first electrodes 13.
The first driving arms 12a are set all on one side of the respective driven arms 10, and are all biased at a first voltage; likewise, the second driving arms 12b are all set on the opposite side of the respective driven arms 10, and are all biased at a second voltage. A driving circuit (not illustrated) is connected to the second electrodes 14a, 14b to apply the first and second voltages and determine, by means of mutual and alternating attraction of the electrodes, an oscillatory rotary motion of the driving mass 3 about the driving axis, at a given oscillation frequency and driving angular velocity {right arrow over (Ω)}a.
The gyroscope 1 further comprises a pair of acceleration sensors with axis parallel to the aforesaid radial direction, and in particular a pair of sensing masses 15a, 15b housed within the through openings 9a, 9b; the sensing masses 15a, 15b have, for example, a generally rectangular shape with sides parallel to corresponding sides of the through openings 9a, 9b, are suspended with respect to the substrate 2a, and are connected to the driving mass 3 via elastic supporting elements 18. The elastic supporting elements 18 depart, for example, from the opposite major sides of each sensing mass in a radial direction. In particular, the elastic supporting elements 18 are rigid with respect to the motion of actuation of the driving mass 3 (in such a way that the sensing masses 15a, 15b will follow the driving mass 3 in the rotary movement), and also enable a linear movement of the respective sensing masses in the aforesaid radial direction. Furthermore, mobile electrodes 20 are coupled to the second sensing masses 15a, 15b, extending, for example, from respective minor sides, in a direction orthogonal to the radial direction; the mobile electrodes 20 form sensing capacitors with plane and parallel plates with respective first and second fixed electrodes 22a, 22b, anchored to the driving mass 3. In particular, each mobile electrode 20 forms a first sensing capacitor C1 with a respective first fixed electrode 22a (for example, the radially more internal one with respect to the center O), and a second sensing capacitor C2 with a respective second fixed electrode 22b (for example, the radially more external one with respect to the center O).
In use, the gyroscope 1 is able to detect an angular velocity {right arrow over (Ω)}z (of yaw), acting about the vertical axis z. In particular, this angular velocity {right arrow over (Ω)}z to be detected generates a Coriolis force {right arrow over (F )}C on the sensing masses 15a, 15b oriented in a radial direction (hence directed as a centripetal force acting on the same masses), causing displacement of the sensing masses and a capacitive variation of the corresponding sensing capacitors C1, C2. The value of the capacitive variation is proportional to the angular velocity {right arrow over (Ω)}z, which can thus be determined in a per-se known manner via a reading circuit, operating according to a differential scheme. In particular, appropriate connections are provided between the fixed electrodes 22a, 22b and the mobile electrodes 20 in such a way that the difference between electrical quantities correlated to the variations of the first and second sensing capacitors C1, C2 are amplified in a differential way.
In particular, in the presence of the Coriolis force due to a yaw angular acceleration acting on the structure, the sensing masses 15a, 15b move in phase opposition in the radial direction (in other words, they displace in opposite senses, or orientations, with respect to the radial direction) so that the differential reading electronics generates an amplified electrical output quantity. Instead, external accelerations acting on the structure in the radial direction (for example, accelerations due to environmental noise) cause a movement in phase of the sensing masses 15a, 15b, which consequently is not read by the reading electronics (given that it does not cause an appreciable output).
Basically, the external accelerations are ideally rejected automatically due to the differential reading. In fact, whereas the useful Coriolis signal tends to unbalance the sensing masses 15a, 15b in opposite radial directions, external accelerations determine variations with the same sign (or sense). By means of the difference between the detection signals generated by the two acceleration sensors it is thus possible to measure the Coriolis contribution and reject the spurious accelerations.
The rotary driving motion also generates a centrifugal acceleration, which acts upon the sensing masses 15a, 15b, substantially in a way similar to the aforesaid Coriolis acceleration (i.e., causing a displacement thereof in opposite directions). However, the centrifugal acceleration causes a contribution at output having a frequency that is twice that of the Coriolis acceleration, and can consequently be appropriately filtered by the reading electronics.
Even though the gyroscope described in the aforesaid patent applications represents a considerable improvement as compared to other structures of a known type, it is not altogether optimized from the standpoint of the electrical characteristics and noise immunity. In particular, in given real operating conditions, it is not perfectly immune to external accelerations (for example, noise accelerations), and also to the effects of the centrifugal acceleration acting on the structure on account of the rotary driving motion.